CODEWITHSUNDEEP
c#
Steinbitglis
Steinbitglis

Reputation: 2614

Can I get a list item by ref return?

I have a list of tuples that I want to modify.

    var myList = new List<(int head, int[] tail)> {
        (0, new[] { 1, 2, 3 }),
        (7, new[] { 8, 9 }   ), };
    var myArray = new (int head, int[] tail)[] {
        (0, new[] { 1, 2, 3 }),
        (7, new[] { 8, 9 }   ), };

    // ...

    // ref var firstElement = ref myList[0];
    ref var firstElement = ref myArray[0];
    firstElement.head = 99;

This works perfectly for arrays, but not for lists. I understand that this is because the indexer doesn't do a ref return; but is there some other way that lists can return their elements by reference, so that I can modify the returned tuple?

This will cause a compiler error "A property or indexer may not be passed as an out or ref parameter":

ref var firstElementList = ref myList[0];

Upvotes: 5

Views: 4476

Answers (2)

Lemm
Lemm

Reputation: 196

You can do this using WeakReference and changing ValueTuple to class:

List<MyClass> myList = new List<MyClass> {
    new MyClass { Head = 0, Tail = new[] { 1, 2, 3 } },
    new MyClass { Head = 7, Tail = new[] { 8, 9 } } };
var firstElement = new WeakReference(myList[0]);
MyClass reference = firstElement.Target as MyClass;
reference.Head = 99;
firstElement.Target = new MyClass { Head = 99, Tail = reference.Tail};
Console.WriteLine(myList[0].Head);

You can try this code here.

Upvotes: 1

Viktor Arsanov
Viktor Arsanov

Reputation: 1613

As I understand it happens because compiler is aware of array and doesn't call indexer for it. While for list it calls indexer and for indexer you cannot use ref(without ref in indexer signature), according to MSDN.

For this

 var firstElement = myArray[0];

 firstElement.head = 99;

Ildasm shows this

ldelem     valuetype [System.Runtime]System.ValueTuple`2<int32,int32[]>

MSDN

I.e. arrays are supported on IL level.

While for list it calls indexer.

 callvirt   instance !0 class [System.Collections]System.Collections.Generic.List`1<valuetype [System.Runtime]System.ValueTuple`2<int32,int32[]>>::get_Item(int32)

And for indexer it works if you put ref to the signature.

E.g.(it is only for demonstration purposes; yep, there should be array instead of single variable, etc, but just to get it compilable)

class Program
{
    static void Main(string[] args)
    {
        var myList = new MyList<(int head, int[] tail)> {
    (0, new[] { 1, 2, 3 }),
    (7, new[] { 8, 9 }   ), };

        ref var firstElement = ref myList[0];
        firstElement.head = 99;
        Console.WriteLine("Hello World!");
    }
}

public class MyList<T> : IEnumerable
{
    private T value;
    public ref T this[int index]
    {
        get
        {
             return ref value;
        }
    }

    public void Add(T i)
    {
        value = i;
    }

    public IEnumerator GetEnumerator() => throw new NotImplementedException();
}

P.S. But when you start implementing your own list implementation(as array list) you will probable notice that it is not worth having ref indexer - imagine you resized the array - created the new one and copied all the data; it means someone might hold non actual reference.

P.P.S Going further, say we creating linked list - nothing wrong will happen on just resize, but imagine we removed an element someone is holding a reference to - it is not possible to understand it doesn't belong to the list any longer.

So, yes, I think they intentionally made List indexer non ref since it doesn't seem to be a good idea having ref return for something that can change.

Upvotes: 3

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